代理设计模式
定义:为其他对象提供一种代理以控制对这个对象的访问。
动态代理使用
java动态代理机制以巧妙的方式实现了代理模式的设计理念。
代理模式示例代码
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public interface Subject { public void doSomething(); } public class RealSubject implements Subject { public void doSomething() { System.out.println( "call doSomething()" ); } } public class ProxyHandler implements InvocationHandler { private Object proxied; public ProxyHandler( Object proxied ) { this .proxied = proxied; } public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable { //在转调具体目标对象之前,可以执行一些功能处理 //转调具体目标对象的方法 return method.invoke( proxied, args); //在转调具体目标对象之后,可以执行一些功能处理 } } |
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import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import sun.misc.ProxyGenerator; import java.io.*; public class DynamicProxy { public static void main( String args[] ) { RealSubject real = new RealSubject(); Subject proxySubject = (Subject)Proxy.newProxyInstance(Subject. class .getClassLoader(), new Class[]{Subject. class }, new ProxyHandler(real)); proxySubject.doSomething(); //write proxySubject class binary data to file createProxyClassFile(); } public static void createProxyClassFile() { String name = "ProxySubject" ; byte [] data = ProxyGenerator.generateProxyClass( name, new Class[] { Subject. class } ); try { FileOutputStream out = new FileOutputStream( name + ".class" ); out.write( data ); out.close(); } catch ( Exception e ) { e.printStackTrace(); } } } |
动态代理内部实现
首先来看看类Proxy的代码实现 Proxy的主要静态变量
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// 映射表:用于维护类装载器对象到其对应的代理类缓存 private static Map loaderToCache = new WeakHashMap(); // 标记:用于标记一个动态代理类正在被创建中 private static Object pendingGenerationMarker = new Object(); // 同步表:记录已经被创建的动态代理类类型,主要被方法 isProxyClass 进行相关的判断 private static Map proxyClasses = Collections.synchronizedMap( new WeakHashMap()); // 关联的调用处理器引用 protected InvocationHandler h; |
Proxy的构造方法
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// 由于 Proxy 内部从不直接调用构造函数,所以 private 类型意味着禁止任何调用 private Proxy() {} // 由于 Proxy 内部从不直接调用构造函数,所以 protected 意味着只有子类可以调用 protected Proxy(InvocationHandler h) { this .h = h;} |
Proxy静态方法newProxyInstance
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public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException { // 检查 h 不为空,否则抛异常 if (h == null ) { throw new NullPointerException(); } // 获得与指定类装载器和一组接口相关的代理类类型对象 Class cl = getProxyClass(loader, interfaces); // 通过反射获取构造函数对象并生成代理类实例 try { Constructor cons = cl.getConstructor(constructorParams); return (Object) cons.newInstance( new Object[] { h }); } catch (NoSuchMethodException e) { throw new InternalError(e.toString()); } catch (IllegalAccessException e) { throw new InternalError(e.toString()); } catch (InstantiationException e) { throw new InternalError(e.toString()); } catch (InvocationTargetException e) { throw new InternalError(e.toString()); } } |
类Proxy的getProxyClass方法调用ProxyGenerator的 generateProxyClass方法产生ProxySubject.class的二进制数据:
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public static byte [] generateProxyClass( final String name, Class[] interfaces) |
我们可以import sun.misc.ProxyGenerator,调用 generateProxyClass方法产生binary data,然后写入文件,最后通过反编译工具来查看内部实现原理。 反编译后的ProxySubject.java Proxy静态方法newProxyInstance
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import java.lang.reflect.*; public final class ProxySubject extends Proxy implements Subject { private static Method m1; private static Method m0; private static Method m3; private static Method m2; public ProxySubject(InvocationHandler invocationhandler) { super (invocationhandler); } public final boolean equals(Object obj) { try { return ((Boolean) super .h.invoke( this , m1, new Object[] { obj })).booleanValue(); } catch (Error _ex) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final int hashCode() { try { return ((Integer) super .h.invoke( this , m0, null )).intValue(); } catch (Error _ex) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final void doSomething() { try { super .h.invoke( this , m3, null ); return ; } catch (Error _ex) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final String toString() { try { return (String) super .h.invoke( this , m2, null ); } catch (Error _ex) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } static { try { m1 = Class.forName( "java.lang.Object" ).getMethod( "equals" , new Class[] { Class.forName( "java.lang.Object" ) }); m0 = Class.forName( "java.lang.Object" ).getMethod( "hashCode" , new Class[ 0 ]); m3 = Class.forName( "Subject" ).getMethod( "doSomething" , new Class[ 0 ]); m2 = Class.forName( "java.lang.Object" ).getMethod( "toString" , new Class[ 0 ]); } catch (NoSuchMethodException nosuchmethodexception) { throw new NoSuchMethodError(nosuchmethodexception.getMessage()); } catch (ClassNotFoundException classnotfoundexception) { throw new NoClassDefFoundError(classnotfoundexception.getMessage()); } } } |
ProxyGenerator内部是如何生成class二进制数据,可以参考源代码。
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private byte [] generateClassFile() { /* * Record that proxy methods are needed for the hashCode, equals, * and toString methods of java.lang.Object. This is done before * the methods from the proxy interfaces so that the methods from * java.lang.Object take precedence over duplicate methods in the * proxy interfaces. */ addProxyMethod(hashCodeMethod, Object.class); addProxyMethod(equalsMethod, Object.class); addProxyMethod(toStringMethod, Object.class); /* * Now record all of the methods from the proxy interfaces, giving * earlier interfaces precedence over later ones with duplicate * methods. */ for (int i = 0; i < interfaces.length; i++) { Method[] methods = interfaces[i].getMethods(); for (int j = 0; j < methods.length; j++) { addProxyMethod(methods[j], interfaces[i]); } } /* * For each set of proxy methods with the same signature, * verify that the methods' return types are compatible. */ for (List<ProxyMethod> sigmethods : proxyMethods.values()) { checkReturnTypes(sigmethods); } /* ============================================================ * Step 2: Assemble FieldInfo and MethodInfo structs for all of * fields and methods in the class we are generating. */ try { methods.add(generateConstructor()); for (List<ProxyMethod> sigmethods : proxyMethods.values()) { for (ProxyMethod pm : sigmethods) { // add static field for method's Method object fields.add(new FieldInfo(pm.methodFieldName, "Ljava/lang/reflect/Method;", ACC_PRIVATE | ACC_STATIC)); // generate code for proxy method and add it methods.add(pm.generateMethod()); } } methods.add(generateStaticInitializer()); } catch (IOException e) { throw new InternalError("unexpected I/O Exception"); } /* ============================================================ * Step 3: Write the final class file. */ /* * Make sure that constant pool indexes are reserved for the * following items before starting to write the final class file. */ cp.getClass(dotToSlash(className)); cp.getClass(superclassName); for (int i = 0; i < interfaces.length; i++) { cp.getClass(dotToSlash(interfaces[i].getName())); } /* * Disallow new constant pool additions beyond this point, since * we are about to write the final constant pool table. */ cp.setReadOnly(); ByteArrayOutputStream bout = new ByteArrayOutputStream(); DataOutputStream dout = new DataOutputStream(bout); try { /* * Write all the items of the "ClassFile" structure. * See JVMS section 4.1. */ // u4 magic; dout.writeInt( 0xCAFEBABE ); // u2 minor_version; dout.writeShort(CLASSFILE_MINOR_VERSION); // u2 major_version; dout.writeShort(CLASSFILE_MAJOR_VERSION); cp.write(dout); // (write constant pool) // u2 access_flags; dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER); // u2 this_class; dout.writeShort(cp.getClass(dotToSlash(className))); // u2 super_class; dout.writeShort(cp.getClass(superclassName)); // u2 interfaces_count; dout.writeShort(interfaces.length); // u2 interfaces[interfaces_count]; for ( int i = 0 ; i < interfaces.length; i++) { dout.writeShort(cp.getClass( dotToSlash(interfaces[i].getName()))); } // u2 fields_count; dout.writeShort(fields.size()); // field_info fields[fields_count]; for (FieldInfo f : fields) { f.write(dout); } // u2 methods_count; dout.writeShort(methods.size()); // method_info methods[methods_count]; for (MethodInfo m : methods) { m.write(dout); } // u2 attributes_count; dout.writeShort( 0 ); // (no ClassFile attributes for proxy classes) } catch (IOException e) { throw new InternalError( "unexpected I/O Exception" ); } return bout.toByteArray(); |
总结
一个典型的动态代理创建对象过程可分为以下四个步骤:
1、通过实现InvocationHandler接口创建自己的调用处理器 IvocationHandler handler = new InvocationHandlerImpl(...);
2、通过为Proxy类指定ClassLoader对象和一组interface创建动态代理类
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Class clazz = Proxy.getProxyClass(classLoader, new Class[]{...}); |
3、通过反射机制获取动态代理类的构造函数,其参数类型是调用处理器接口类型
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Constructor constructor = clazz.getConstructor( new Class[]{InvocationHandler. class }); |
4、通过构造函数创建代理类实例,此时需将调用处理器对象作为参数被传入
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Interface Proxy = (Interface)constructor.newInstance( new Object[] (handler)); |
为了简化对象创建过程,Proxy类中的newInstance方法封装了2~4,只需两步即可完成代理对象的创建。
生成的ProxySubject继承Proxy类实现Subject接口,实现的Subject的方法实际调用处理器的invoke方法,而invoke方法利用反射调用的是被代理对象的的方法(Object result=method.invoke(proxied,args))
美中不足
诚然,Proxy已经设计得非常优美,但是还是有一点点小小的遗憾之处,那就是它始终无法摆脱仅支持interface代理的桎梏,因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图,它们已经注定有一个共同的父类叫Proxy。Java的继承机制注定了这些动态代理类们无法实现对class的动态代理,原因是多继承在Java中本质上就行不通。有很多条理由,人们可以否定对 class代理的必要性,但是同样有一些理由,相信支持class动态代理会更美好。接口和类的划分,本就不是很明显,只是到了Java中才变得如此的细化。如果只从方法的声明及是否被定义来考量,有一种两者的混合体,它的名字叫抽象类。实现对抽象类的动态代理,相信也有其内在的价值。此外,还有一些历史遗留的类,它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种,不得不说是一个小小的遗憾。但是,不完美并不等于不伟大,伟大是一种本质,Java动态代理就是佐例。
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